DETROIT (AP) — Ford is recalling more than 550,000 pickup trucks in the U.S. because the transmissions can unexpectedly downshift to first gear no matter how fast the trucks are going.

The recall covers certain F-150 pickups from the 2014 model year. Ford’s F-Series pickups are the top-selling vehicles in the U.S.

Ford says in documents posted Tuesday by U.S. safety regulators that the downshifting can cause drivers to lose control of the trucks, increasing the risk of a crash.

The recalls come after U.S. auto safety regulators in March began investigating complaints that more than 540,000 Ford pickup trucks from 2014 can abruptly downshift to a lower gear and increase the risk of a crash.

Documents say the problem is caused by a lost signal between a transmission speed sensor and the powertrain control computer. There also could be corrosion and problems with connector pins.

Dealers will update the powertrain control software at no cost to owners, who will be notified by letters in early July.

Ford says in documents posted Tuesday by the National Highway Traffic Safety Administration that it has 396 warranty and field reports and 124 customer complaints about the problem, covering 482 trucks.

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The Dearborn, Michigan, automaker says it knows of 130 complaints to the government, with 52 alleging that rear wheels locked up or that drivers lost control of the trucks. Two of the complainants reported injuries and one reported a crash that could have been caused by the problem, Ford said.

NHTSA said it started the probe in March after getting complaints about sudden downshifts in the trucks’ automatic transmissions.

The agency is looking into whether those trucks should have been included in previous recalls for the problem.

Certain F-150 pickups from the 2011 and 2012 model years were recalled for the same problem in 2016. Ford later added 2013 models to the recall.

The company said it’s working with NHTSA to support the investigation.